On Sept. 11, 2001, nineteen people, using knives and boxcutters, hijacked four transcontinental airline flights and flew them toward America's financial and political capitals of New York and Washington.

Three reached their targets: The first two struck the towers of the World Trade Center, causing their collapse within hours, and the third struck the Pentagon.

The pilot of the fourth plane crashed it into a field in Pennsylvania after the passengers rose up against the hijackers.

American air defense fighters scrambled against the threats but were ultimately misdirected and had no effect on the attacks.

In New York, where most of the nearly 3,000 fatalities took place, the dead included 343 members of the New York Fire Department, including the chief, 37 members of the Port Authority Police Department and 23 members of the New York Police Department. Of between 16,400 and 18,800 civilians in the towers on 9-11, 2,152 died; the rest were successfully evacuated. About 95 percent of the dead civilians worked in or above the floors where the planes impacted.

The third plane came in low over the Arlington National Cemetery and struck the western face of the Pentagon, penetrating through three of the building's five rings. All 64 people on the plane, including the five hijackers, were killed; 125 in the Pentagon also died.

U.S. suspicion immediately centered on al-Qaeda as well as Iraq, but intercepted terrorist communcations pinpointed Osama bin Laden's network as responsible. Bin Laden and other al-Qaeda leaders later took credit for the operation.

CIA officers entered Afghanistan, al-Qaeda's home, almost immediately, to reestablish contacts with fighters opposed to al-Qaeda's allies, the Taliban, which controlled most of the country. The Taliban was ousted from power within a few months, but many al-Qaeda leaders, including bin Laden, escaped to Pakistan and Iran.

One of those leaders, Khalid Shaikh Mohammed, was the mastermind of the attacks. He proposed such an operation to bin Laden in 1996; in late 1998 or early 1999, bin Laden approved it, and Mohamed Atef, Khalid Shaikh Mohammed and bin Laden began assembling the hijacker teams.

Central to the plot was the "Hamburg Cell" a group of well-educated Muslims who had become radicalized while living in Germany. The three members -- Atta, Shehhi and Jarrah -- who could obtain U.S. visas piloted three of the four aircraft hijacked on 9-11.

Also involved were two veteran al-Qaeda members, Mihdhar and Hazmi, who tried and failed to become pilots. They ultimately served as muscle on American Airlines 77, which crashed into the Pentagon. It was piloted by Hani Hanjour, another al-Qaeda recruit with flying experience who was added to the plot as it progressed.

Providing the bulk of the muscle were 13 young hijackers, 12 of them Saudi, who were gathered and trained in Afghanistan. Many came from the poorer parts of Saudi Arabia; most were unmarried and had no more than a high school education. It is believed they were not briefed on details of the plot until they reached the United States.

Other potential hijackers, comprising a slate of so-called "20th hijackers," either quit the plot or were not allowed into the United States.2,3,4

Possible

The plan, as originally conceived by Khalid Shaikh Mohammed, involved hijacking 10 planes, 9 of which would be used as missiles. In addition to the World Trade Center and Pentagon, additional targets included CIA and FBI headquarters, nuclear power plants, and the tallest buildings in California and Washington State. KSM himself would land the 10th plane at a U.S. airport, kill all adult males on board, and then deliver a speech to the news media castigating U.S. support for Israel, the Philippines and Arab governments.

Bin Laden pared down the plan to its final size.

Attash as claimed that many of the the Saudi "muscle" hijackers tried to reach the fighting in Chechnya in 1999, but were unable to reach the front. They then went to Afghanistan for training, where they heard bin Laden speak and volunteered to become suicide operatives.1